1. Field of the Invention
The present invention relates to a switching regulator control circuit capable of readily switching an operation of a step up/down switching regulator at a step-up time and a step-down time in accordance with a relationship among an input voltage, an output voltage, and an output current, and to a switching regulator having the switching regulator control circuit.
2. Description of the Related Art
Shown in FIG. 2 is an example of conventional step-up/down switching regulator circuits. A switching regulator control circuit 1, a step-down switch 2, a step-up switch 6, a diode 7, a diode 4, and a coil 3 are connected to each other as shown in FIG. 2. In the circuit, a portion including the step-down switch 2, the diode 7, and the coil 3 corresponds to a conventional step-down switching regulator circuit. Another portion including the coil 3, the step-up switch 6, and the diode 4 corresponds to a conventional step-up switching regulator circuit. The switching regulator circuit 1 detects a voltage at a terminal Vout. When a step-up operation is required according to the relationship among the input voltage, the output voltage, and the output current, the switching regulator circuit 1 controls the step-up switch 6 so as to step up a voltage level of the terminal Vout to a desired voltage level. In this situation, the step-down switch 2 is always kept on. On the other hand, when a step-down operation is required according to the relationship among the input voltage, the output voltage, and the output current, the switching regulator circuit 1 controls the step-down switch 2 so as to step down the voltage level of the terminal Vout to a desired voltage level. In this situation, the step-up switch 6 is always kept off.
Subsequently, the operation of the switching regulator control circuit 1 will be described in more detail with reference to FIG. 3. A voltage Va resulting from dividing an output voltage Vout by means of a dividing resistor 11 and a dividing resistor 12 is inputted to a non-inverting input terminal of an error amplifier circuit 13. A reference voltage Vref that is outputted from a reference voltage circuit 10 is inputted to an inverting input terminal of the error amplifier circuit 13. The error amplifier circuit 13 outputs a result of amplifying a difference between the voltage Va and the reference voltage Vref as a voltage Verr.
A chopping wave Vosc that is outputted from a chopping wave oscillating circuit 14 is inputted to a non-inverting input terminal of a step-down comparator circuit 16 and then compared with the voltage Verr that is inputted to the inverting input terminal. When the voltage Verr falls within an amplitude range of the chopping wave Vosc, a rectangular wave VcompD is outputted from the step-down comparator circuit 16, and the switching regulator control circuit 1 controls the step-down switch 2 and conducts the step-down operation. In this situation, the step-up switch 6 is off.
The chopping wave Vosc that is outputted from the chopping wave oscillating circuit 14 is inputted to a non-inverting input terminal of a step-up comparator circuit 17 as a chopping wave Vosc1 that has been converted into a voltage by means of a level shifter 15. The chopping wave Vosc is then compared with the voltage Verr that is inputted to the inverting input terminal. When the voltage Verr falls within an amplitude range of the chopping wave Vosc1, a rectangular wave VcompU is outputted from the step-up comparator circuit 17, and the switching regulator control circuit 1 controls the step-up switch 6 and conducts the step-up operation. In this situation, the step-down switch 2 is on.
In addition, the error amplifier circuit 13 is held stable by means of a phase compensating circuit 38 (for example, refer to JP 2003-180072 A).
However, in the conventional step-up/down switching regulator, the number of phase compensating circuits for stabilizing the error amplifier circuit is one. Therefore, in a general purpose switching regulator that copes with various input and output conditions, it is difficult to set a constant of the phase compensating circuit for stabilizing the error amplifier circuit over an overall output voltage range from step-up to step-down. Also, there is a case where the phase compensating constant to be set is different between step-up and step-down depending on an intended purpose of the step-up/down switching regulator, and there arises such a problem that the use conditions are limited.